This invention relates to a stored energy system used for the propulsion of a motor vehicle, and in particular to a system wherein the prime mover constitutes a free piston engine-pump. Such stored energy system typically uses an energy accumulator which acts to decouple the instantaneous power applied to the wheels from the power developed by the prime mover. Such energy accumulator is typically a pneumatically pressured hydraulic accumulator. Such stored energy propulsion system is illustrated in FIG. 1 hereof (see also Applicant's U.S. Pat. No. 4,087,205). In this prior art system, pressurized fluid output from the free piston engine-pump unit is supplied through appropriate circuitry to a hydraulic motor (such as a swash-plate motor) which in turn drives the vehicle wheels such as through an appropriate change-speed gear arrangement. The fluid circuitry which couples the output of the engine-pump to the hydraulic motor typically incorporates a large-capacity accumulator for absorbing flow pressure energy during the normal or steady-state operation of the vehicle, and for adding energy back into the system when the vehicle demands high engine power levels for shorter periods of time, whereby the power capacity of the prime mover (i.e., the free piston engine-pump) can be sized more closely to meet the steady-state demand rather than the maximum peak demand.
With this known system, however, wherein only a large-capacity accumulator is employed, the ability to modulate the working pressure as supplied to the drive motor, in response to power demand, is restricted in that any significant pressure modulation can generally be accomplished only over a rather long time interval such as in the order of several seconds inasmuch as the large capacity of the accumulator necessarily results in substantial inertia which prevents rapid pressure changes or fluctuations over short time intervals.
Accordingly, the present invention relates to a stored energy-type propulsion system for a vehicle, specifically a system employing a free piston engine-pump, which system has the capability of responding to peak power demands by using a large-capacity accumulator such as exists in the prior art systems, but which improved system also has the capacity of responding to sudden changes in power demand such as changes which demand a sudden pressure increase in the system within an extremely small time interval such as in the order of about 0.1 second, particularly when such power demand occurs during operation of the vehicle under such conditions that the power demand is still significantly less than the peak power demand requirement. This improved system hence overcomes disadvantages associated with the prior art system in providing increased engine performance and responsiveness under substantially all driving conditions.
In the improved stored energy propulsion system of this invention, the free piston engine-pump is of the double-acting type and functions as the prime mover, and is hydraulically connected to the drive motor for the vehicle, which drive motor in the conventional manner is responsive to the vehicle demand as controlled by the vehicle accelerator pedal. The hydraulic connection between the engine-pump and the drive motor includes a large-capacity pressure accumulator. This accumulator is controlled by valving which is maintained closed under normal operating and steady-state operating conditions of the vehicle to maintain pressure in this accumulator at close to the maximum pressure operating level (such as about 5000 psi) of the system. The valving to this large accumulator is opened only in response to a signal which senses that the vehicle, when operating at a normal state, has demanded substantially peak capacity such as due to the vehicle encountering a steep hill or the driver under a highway condition pressing the accelerator suddenly down to almost the full extent to permit sudden passing of another vehicle. Under such condition, the large accumulator is permitted to communicate with and supply stored high-pressure fluid into the system to provide additional driving power to the drive motor, whereby the engine-pump will continue to operate at its steady-state level and, after the engine returns to its steady-state level, the valving will again close off the large accumulator except that it does function in a manner equivalent to a one-way check valve so as to permit recharging of the large accumulator. In addition, the hydraulic system which connects between the engine-pump and the drive motor also has an auxiliary small-capacity pressure accumulator connected in fluid communication therewith. This auxiliary accumulator has a size which is very small in relationship to the large accumulator such as in the neighborhood of between about 0.1 to about 0.05 the capacity of the large accumulator. This auxiliary accumulator has valving associated therewith which is normally open so that this accumulator is normally in continuous communication with the main pressure system to effectively act as a cushion so as to minimize pressure pulses due to cyclic operation of the engine-pump. In addition, the engine-pump is preferably operated in an ON-OFF system cycle which results in the engine-pump being operated for a selected number of cycles (such as about 10 or more cycles over a time interval of about 1 second, for example) during the ON part of the system cycle, followed by the engine-pump being stopped for an equivalent time duration during the OFF part of the system cycle, and during this OFF part of the system cycle the auxiliary accumulator supplies pressure fluid into the system for driving the drive motor. In addition, when the engine has need for a sudden pressure increase of short duration due to sudden depression of the accelerator pedal, then a signal closes off the valve of the auxiliary accumulator whereby it is effectively isolated from the system, and hence the pressure in the system can suddenly increase (such as over a short time interval of about 0.01 second) due to the operation of the engine-pump and specifically its excess capacity, thereby providing a substantially instantaneous increase in the power output of the driving motor. The auxiliary accumulator preferably remains in constant communication with the hydraulic system through a small orifice which permits pressure fluid to be supplied to the auxiliary accumulator but does not interfere with the sudden pressure increase within the hydraulic system. As soon as this sudden demand for power is eliminated, such as the vehicle returning to its normal or steady state, then the signal to this latter valve is changed and permits the valve to reopen. The auxiliary accumulator thus returns to the performance of its normal functions of cushioning the pressure pulses within the system during the ON part of the system cycle and supplying pressure fluid to the drive motor during the OFF part of the system cycle. With this arrangement, the pressure buildup in the system through use of the auxiliary accumulator can occur within a short time interval, such as a maximum time interval of about 1 second, which speed of response is not possible with the large accumulator. The large accumulator remains isolated from the system and becomes operative only when peak power is demanded.
Other objects and purposes of the invention will be apparent to persons familiar with systems of the above type upon reading the following specification and inspecting the accompanying drawings.